Enzymes are biomolecules (typically proteins) that catalyze chemical reaction. In fact, enzymes are known to play a part in literally thousands of biochemical reactions. Enzymes generally function by action on a specific substrate to form a specific product, and the underlying reactions are dramatically hindered or completely impossible in the absence or inhibition of the necessary enzyme. Enzyme inhibitors are compounds or molecules that interact with enzymes in some fashion so as to inhibit the normal activity of the enzyme. For example, the binding of an inhibitor can stop a substrate from entering the active site of the enzyme and/or hinder the enzyme from catalyzing its specific reaction. The level of inhibition can vary from a mild decrease in effectiveness of the enzyme to a complete ceasing of enzyme function. Given the broad activity of various enzymes in living organisms, it is not surprising that many enzyme inhibitors have been developed for pharmaceutical applications. Medicinal enzyme inhibitors are often judged based on specificity (i.e., lack of binding to other proteins) and potency (i.e., the dissociation constant, which indicates the concentration needed to inhibit the specified enzyme). High specificity and potency are desired to limit drug side effects and ensure low toxicity.
One group of enzyme inhibitors that has shown great usefulness is the so-called “antifolates”. Folic acid is a water-soluble B vitamin known by the systematic name N-[4(2-amino-4-hydroxy-pteridin-6-ylmethylamino)-benzoyl]-L(+)-glutamic acid and having the structure provided below in Formula (1).
As seen in Formula (1), the folic acid structure can generally be described as being formed of a pteridine ring, a para-aminobenzoic acid moiety, and a glutamate moiety. Folic acid and its derivatives are necessary for metabolism and growth, particularly participating in the body's synthesis of thymidylate, amino acids, and purines. Derivatives of folic acid, such as naturally occurring folates, are known to have biochemical effects comparable to folic acid. Folic acid itself is derivatized via hydrogenation, such as at the 1,4-diazine ring, or being methylated, formaldehydylated, or bridged, wherein substitution is generally at the N5 or N10 positions. Folates have been studied for efficacy in various uses including reduction in severity or incidence of birth defects, heart disease, stroke, memory loss, and age-related dementia.
Antifolate compounds are structurally similar to folic acid and function to disrupt folic acid metabolism. A review of antifolates is provided by Takamoto (1996) The Oncologist, 1:68-81, which is incorporated herein by reference. One specific group of antifolates, the so-called “classical antifolates,” is characterized by the presence of a folic acid p-aminobenzoylglutamic acid side chain, or a derivative of that side chain. Another group of antifolates, the so-called “nonclassical antifolates,” are characterized by the specific absence of the p-aminobenzoylglutamic group. Because antifolates have a physiological effect that is opposite the effect of folic acid, antifolates have been shown to exhibit useful physiological functions, such as the ability to destroy cancer cells by causing apoptosis.
An intact folate enzyme pathway is important to maintain de novo synthesis of the building blocks of DNA, as well as many important amino acids. Antifolate targets include the various enzymes involved in folate metabolism, including (i) dihydrofolate reductase (DHFR); (ii) thymidylate synthase (TS); (iii) folylpolyglutamyl synthase (FPGS); (iv) glycinamide ribonucleotide formyltransferase (GAR); and (v) aminoimidazole carboxamide ribonucleotide transformylase (AICAR Tfase).
The reduced folate carrier (RFC), which is a transmembrane glycoprotein, plays an active role in the folate pathway transporting reduced folate into mammalian cells via the carrier mediated mechanism (as opposed to the receptor mediated mechanism). The RFC also transports antifolates, such as methotrexate. Thus, mediating the ability of RFC to function can affect the ability of cells to uptake reduced folates.
Further folic acid derivatives have also been studied in the search for antifolates with increased metabolic stability allowing for smaller doses and less frequent patient administration. For example, a dideaza (i.e., quinazoline-based) analog has been shown to avoid physiological hydroxylation on the pteridine ring system. Furthermore, replacement of the secondary amine nitrogen atom with an optionally substituted carbon atom has been shown to protect neighboring bonds from physiological cleavage.
The multiple enzymes involved in folic acid metabolism within the body present a choice of inhibition targets for antifolates. In other words, it is possible for antifolates to vary as to which enzyme(s) they inhibit. For example, some antifolates inhibit primarily DHFR, while other antifolates inhibit primarily TS, GAR, FPGS, or AICAR Tfase, while still other antifolates inhibit combinations of these enzymes.
Given the usefulness of enzyme inhibiting compounds in treating various conditions and diseases, it would be beneficial to have new compounds exhibiting such enzyme inhibition. The present invention meets this need.